Question

why yield point phenomenon is not observed in aluminum

Answer 1

Steel shows this kind of behavior due to the presence of large interstitial atoms such as Carbon and Nitrogen. These atoms are much larger than the interstitial sites in iron, thus producing compressive stress on iron atoms adjacent to them. As a result of this mismatch, C & N atoms diffuse into the dislocation sites as these sites have larger space to accommodate these atoms. This C & N rich atmosphere is called Cottrel Atmosphere. Due to the presence of these atoms it becomes harder for dislocations to move. So, a higher stress is required to break this atmosphere and to move the dislocations. Thus Upper Yield Point appears. Once the dislocations have been freed, it needs lower stress to move them, thus Lower Yield Point appears.

In pure Aluminum there are no such interstitial atoms like C or N. So movement of dislocation is comparatively easy and they show continuous yielding behavior.